Electronic
Nose
For ages,
the human nose has been an important tool in assessing
the quality of many products, food products being good
examples. While all others parts of production processes,
including these of the food industry, were getting more
and more automated, there was still no 'objective' means
for using the 'subjective' information confined in the
smell of products. This changed in 1982, when Persaud
and Dodd introduced the concept of an electronic nose.
They proposed a system, comprising an array of essentially
non-selective sensors and an appropriate pattern recognition
system. Much research was done in order to find new
and more diverse sensors, and to date there are several
companies offering ready-to-use electronic noses.
An "electronic or artificial
nose" is an instrument, which comprises a sampling
system, an array of chemical gas sensors with differing
selectivity, and a computer with an appropriate pattern-classification
algorithm, capable of qualitative and/or quantitative
analysis of simple or complex gases, vapors, or odors.
An "electronic tongue" uses an array of liquid
sensors. The artificial "chemical" senses
include taste and olfaction.
This writeup is about the electronic
nose and specifically about electronic noses based on
electrochemical sensors ("electrochemical nose"),
the class with which we are most experienced. The entire
genus of electronic noses includes those with conductive
polymer, polymer composite, quartz microbalance, surface
acoustic wave, calorimetric, and other classes of sensors.
In this discussion, we will simply refer to an "E-nose"
to indicate artificial olfaction, since many modern E-noses
are constructed with more than one class of sensor in
them. These latter instruments are said to employ "heterogeneous"
sensor arrays. Many sources of multi-parameter chemical
data including infrared spectrometers, gas chromatographs,
and mass spectrometers have been used to identify odors
and therefore called E-noses. So, even though the above
definition is broad, it may not be broad enough to describe
this entire field of technology.
Biological nose
One cannot discuss the "electrochemical
nose" without first discussing the human or biological
nose and the factors that led to the development of
artificial olfaction technology. Of all the five senses,
olfaction uses the largest part of the brain and is
an essential part of our daily lives. Indeed, the appeal
of most flavors is more related to the odor arising
from volatiles than to the reaction of the taste buds
to dissolved substances. Our olfactory system has evolved
not only to enhance taste but also to warn us of dangerous
situations. We can easily detect just a few parts per
billion of the toxic gas hydrogen sulfide in sewer gas,
an ability that can save our life. Olfaction is closely
related to the limbic or primitive brain, and odors
can elicit basic emotions like love, sadness, or fear.
In fact life-saving nauseous revulsion is at least as
ancient and universal in the mammalian world as the
animal that takes most exquisite advantage of its existence,
"Mephitis mephitis", the striped skunk!
How does it work ?
The operation of the device pumps
the sample and records the signals from four electrochemical
sensors at four inlet operating conditions over about
a minute. This produces 16 responses and these are recorded
for four substances: methyl isocyanate, hydrogen cyanide,
nitrogen dioxide, and carbon monoxide. These signals
were recorded at several different concentrations in
the working range of the sensors (about 0 to 1000 parts
per million). One can tell the amount of the material
by the strength of the signals $150; the stronger the
responses, the higher the concentration.
E-nose sensors
1. Amperometric
2. Chemoresistor
3. Chemocapacitor
4. Optical
5.Gravimetric
6. Polymer
Applications
1. Medical: To detect lung cancers, vaginal infections,
TB detection
2. Scientific: To detect ammonia gas leakage on space
board station.
3. Military: To detect land mines
4. Industrial: To detect freshness of fish, food quality
5. Others
Future trends
The next generation of e-nose points
out accuracy and compactness that's why scientists are
researching on new version of e-nose, that is zNose.
Its main feature is that it uses only one physical
sensor, instead of sensors array.
Advantages of zNose
- The zNose is fast, able to speciate and measure
the chemical composition of collected vapors in 10
seconds.
- Easy to learn and use Win-dows® Software.
- The zNose can perform over 300 measurements in an
8 hour shift
- zNose is sensitive, parts per billion and even parts
per trillion for certain aromatic compounds
- zNose provides 2-dimensiional high resolution olfactory
images based upon product chemistry.
- Principal component analysis, virtual chemical sensors
- New biofeedback tool and documentation for sensory
panels
The electronic nose is a prime example of successful
application of neural network technology. It uses many
of the rudimentary concepts from bio-logical olfaction
including the sniffing, chemical detection and odor
recognition process. The range of simple gas sensing
devices available is already quite diverse.
Source: Invention
Intelligence, September - October 2006
